Purine nucleoside phosphorylase (PNP) deficiency is associated with defective T cell and normal B cell function. We will investigate the biochemical mechanisms relating the enzyme deficiency to the immune defect. We will test our hypothesis that the selectivity of the T cell defect is caused by the preferential metabolism of deoxyguanosine to deoxyGTP in T and not B cells. We will evaluate alternative mechanisms involving: differential catabolism of nucleotides in T and B cells; altered transmethylation; interruption of protein or RNA synthesis; and variation in intracellular cyclic nucleotides. In this proposal we will use B and T lymphoblastoid cells, PNP B LCLs, PNP fibroblasts, and will isolate T cell depleted B lymphocytes, T lymphocytes and T cell subsets. We will synthesize inhibitors of PNP to simulate the enzyme deficiency in the LCLs, B and T cells, and T cell subsets. The PNP inhibited lymphocyte populations will provide the most appropriate in vitro model of PNP deficiency. This research will include cell growth experiments, incubation with radiolabeled nucleosides, the analysis of nucleotides by HPLC and measurement of enzyme activities. Based on our observation that T cells possess a higher deoxyguanosine kinase activity than B cells, we will synthesize inhibitors of this kinase to block the accumulation of cytotoxic deoxyGTP as a possible means of therapy for PNP deficient patients. The regulation of T cell function will be investigated in an in vivo canine model of PNP deficiency. This research will permit a novel approach to the regulation of T cell lymphopoiesis and will enable the design of agents to specifically interrupt T cell fuction that will be applicable in transplantation and cancer therapy. The information gained will allow the development of agents for the treatment of PNP deficiency.